1. Field of the Invention
The claimed invention generally relates to image processing and, more particularly, to an image processing apparatus and an image processing method for correcting the tint of a printer, and a storage medium storing a program for generating an image processing parameter.
2. Description of the Related Art
As the performance of an electrophotographic apparatus has been improved in recent years, an electrophotographic apparatus realizing the same image quality as that of a printing machine has appeared. However, the issue that remains is that a variation amount of colors is larger than that of the printing machine due to instability specific to electrophotography.
Therefore, a conventional electrophotographic apparatus includes a calibration technique for generating a one-dimensional look up table (LUT) for gradation correction corresponding to each of cyan, magenta, yellow, and black (hereinafter, referred to as C, M, Y, and K) toners. The LUT is a table representing output data corresponding to input data separated with specific interval, and can represent a non-linear characteristic that cannot be expressed in an arithmetic expression.
A method for generating the one-dimensional LUT will be described. A printer outputs a chart including data, which differ in gradation, corresponding to the C, M, Y, and K toners. A scanner or a colorimeter then reads the output chart, to acquire a density value. The read density value is compared with a target prepared in advance in the printer, to generate a one-dimensional LUT (1D-LUT) for correction independently for each of the C, M, Y, and K toners.
However, a non-linear difference occurs for a “mixed color” particularly in the electrophotographic apparatus even if the 1D-LUT corrects a gradation characteristic of each color, so that a tint is not easy to ensure. The “mixed color” is a color using a plurality of toners in gray or the like using red (R), green (G), blue (B), and CMY.
As a solution to the above-mentioned issue, a “mixed color” calibration technique is discussed in Japanese Patent Application Laid-Open No. 2005-175806. The outline of the “mixed color” calibration technique will be described below. A chart generated in the “mixed color” is output, to obtain a measured value using the scanner or the colorimeter. The obtained “measured value” is then compared with a “target value”, to generate a corrected value.
The “target value” represents a characteristic of a mixed color (hereinafter, referred to as a mixed color characteristic) of an image output at any timing of the electrophotographic apparatus. The “measured value” represents the current mixed color characteristic of the electrophotographic apparatus. A difference between the “measured value” and the “target value” is obtained, to generate the corrected value so that the mixed color characteristic of the electrophotographic apparatus is brought as close to a state of the “target value” as possible.
The user can register the “target value” using a user interface (UI) at any timing. Calibration is performed using the “target value” registered by the user so that the mixed color characteristic of the electrophotographic apparatus can be continuously maintained at timing designated by the user.
If the user registers the “target value”, a sheet used at the time of the registration and a sheet used during the calibration are preferably the same. If the sheets differ, a particular issue is that they differ in “paper white” serving as a white color of paper itself. The “paper white” can be acquired at a quantitative value such as L*a*b. The L*a*b is one of device-independent color spaces determined by International Commission on Illumination (CIE), where “L*” represents a luminance, and “a*” and “b*” represent a hue and a saturation, respectively.
An effect of the difference in the “paper white” can be reduced using a known technique called “white point correction” by acquiring the above-mentioned L*a*b.
If a glossy “coated sheet” is used for mixed color calibration, however, a measurement result deviates depending on its “glossiness”, resulting in a deteriorated accuracy of the calibration.
A sheet used during calibration by the conventional electrophotographic apparatus is a “plain sheet”. If a sheet used at the time of registration and the sheet during the calibration differ, the two types of sheets differ in “paper white” while being substantially the same in glossiness.
On the other hand, the “coated sheet” differs in not only “paper white” but also a “glossiness” depending on its brand. A common index such as L*a*b has not been established for the “glossiness”. Moreover, the “glossiness” is significantly difficult for the user to quantitatively determine by seeing the sheet.
However, a measurement device such as a scanner or a colorimeter is greatly affected by the “glossiness”. More specifically, even if electrophotographic apparatuses having the same mixed color characteristic respectively output charts, a measurement result of a “mixed color” differs when the charts differ in the “glossiness”. While a range affected by the “paper white” is centered on a color having a high luminance (e.g., a white color), a range affected by the “glossiness” affects a color in general. Accordingly, the effect is not easy to reduce by using “white point correction” of a known technique.